Project Summary: Spinal cord injury (SCI) is a devastating neurological disorder that often impairs the daily function of patients for their entire life. One of the major obstacles in treating SCI is how to restore the lost neuronal functions. Despite decades of research efforts, current strategies including stem cell transplantation have not resulted in a successful clinical therapy. Therefore, there is an urgent need to develop novel technology to treat SCI. This research team has recently developed an innovative approach to reprogram reactive astrocytes into functional neurons in situ for brain repair by injecting viral particles expressing a single neural transcription factor NeuroD1 into the adult mouse cortex (Guo et al., Cell Stem Cell, BEST of 2014 article). This proposal will introduce this cutting-edge in vivo reprogramming technology into SCI. In particular, the PI proposes to convert endogenous reactive astrocytes into functional neurons by forced expression of a neuronal specific microRNA (miRNA) miR-124. Most of current in vivo reprogramming studies including the one from this team have been done by using viral vectors expressing neurogenic transcription factors; and in vivo neuronal conversion by miRNAs has not been reported. MiRNAs are small non-coding RNAs that play pivotal roles during neural development and diseases. The miRNA function could be potent in that one miRNA may regulate many target genes through the unique imperfect base-pairing mechanism. Furthermore, their small size (~22 nucleotides) makes them attractive for therapeutic application since they may easily penetrate tissues and be taken up by target cells. MiR-124 plays critical roles in neurogenesis, neuronal differentiation and maturation, which makes it an ideal candidate for neuronal reprogramming. Therefore, in this proposal, the PI will test the hypothesis that forced expression of miR-124 can convert reactive astrocytes into functional neurons in the injured spinal cord, and that converted neurons can integrate into the local neuronal circuitry and promote functional recovery after SCI. The PI proposes two specific aims: 1) To determine conversion of reactive astrocytes into neurons by miR-124 after SCI; 2) To determine functional integration of miR-124-converted neurons and their effects on animal's behavior after contusive SCI. Completion of the proposed study here will show feasibility of miR-124-mediated glia-neuron conversion in vivo and lay out foundation for therapeutic application of this small RNA molecules as a synthetic drug in the future. In addition, the success of this proposal will potentially lead to a novel therapeutic treatment for SCI as well as other neurological diseases such as traumatic brain injury (TBI) and amyotrophic lateral sclerosis (ALS).